Merge branch 'master' of github.com:jboner/akka

akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala

@@ -0,0 +1,20 @@
+/**
+ * Copyright (C) 2009-2011 Scalable Solutions AB <http://scalablesolutions.se>
+ */
+
+package akka.actor
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+import akka.dispatch._
+
+class ChannelSpec extends WordSpec with MustMatchers {
+  "A Channel" must {
+    "be contravariant" in {
+      val ap = new ActorPromise(1000)
+      val p: Promise[Any] = ap
+      val c: Channel[Any] = ap
+      val cs: Channel[String] = c
+    }
+  }
+}

akka-actor/src/main/scala/akka/actor/Channel.scala

@@ -8,14 +8,11 @@ package akka.actor
  * Abstraction for unification of sender and senderFuture for later reply.
  * Can be stored away and used at a later point in time.
  *
- * Channel cannot be contravariant because of Future providing its value in
- * covariant position.
- *
  * The possible reply channel which can be passed into ! and safe_! is always
  * untyped, as there is no way to utilize its real static type without
  * requiring runtime-costly manifests.
  */
-trait Channel[T] {
+trait Channel[-T] {
 
   /**
    * Scala API. <p/>
@@ -105,7 +102,7 @@ trait Channel[T] {
  * i.e. ! is not guaranteed to fail (e.g. NullChannel would be a
  * counter-example).
  */
-trait AvailableChannel[T] { self: Channel[T] ⇒
+trait AvailableChannel[-T] { self: Channel[T] ⇒
   def safe_!(msg: T)(implicit channel: UntypedChannel = NullChannel): Boolean = {
     if (isUsable) {
       try {

akka-docs/scala/actors.rst

@@ -266,7 +266,7 @@ Reply using the channel
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 If you want to have a handle to an object to whom you can reply to the message, you can use the ``Channel`` abstraction.
-Simply call ``self.channel`` and then you can forward that to others, store it away or otherwise until you want to reply, which you do by ``Channel ! response``:
+Simply call ``self.channel`` and then you can forward that to others, store it away or otherwise until you want to reply, which you do by ``channel ! response``:
 
 .. code-block:: scala
 
@@ -274,6 +274,17 @@ Simply call ``self.channel`` and then you can forward that to others, store it a
       val result = process(request)
       self.channel ! result
 
+The :class:`Channel` trait is contravariant in the expected message type. Since
+``self.channel`` is subtype of ``Channel[Any]``, you may specialise your return
+channel to allow the compiler to check your replies::
+
+  class MyActor extends Actor {
+    def doIt(channel: Channel[String], x: Any) = { channel ! x.toString }
+    def receive = {
+      case x => doIt(self.channel, x)
+    }
+  }
+
 .. code-block:: scala
 
   case request =>